1. Field of the Invention
The present invention relates to a mask and a method of forming a pattern, and more particularly, to a mask having an assist pattern and a method of forming a pattern by using the same mask.
2. Description of the Prior Art
With the trend of miniaturization of the electronic products and peripherals, research about thin structures and high integration of the semiconductor devices have become the essential subjects and developing aspects in the industry, and the lithography technology plays an important role to determine the performances of the semiconductor devices.
In semiconductor manufacturing processes, the integrated circuit layout is first designed and formed as a mask pattern. The mask pattern is then proportionally transferred to a photoresist layer disposed on the semiconductor wafer through an exposure process followed by a development process. Subsequently, a corresponding etching process is performed in order to manufacture the semiconductor devices on the semiconductor wafer. With the demand of increasing integration of semiconductor devices, the design rule of line width and spaces between lines or devices becomes finer. However, due to the optical proximity effect (OPE), the width is subject to optical limitations. To obtain the fine-sized devices, the pitch i.e. the interval between transparent regions in a mask is scaled down along with the device size. However, if the pitch is scaled down to a specific range, for example, equal to or smaller than half of the wavelength of light used in the exposure process, when the light passes through the mask, diffraction and interferences may occur, and the resolution of the mask pattern transferred onto the photoresist layer would be affected; in other words, due to the OPE, the deviation of the transferred pattern such as rounded right-angle corners, shortened line-ends, or increase/decrease of line widths would occur. Furthermore, when light passes through the transparent regions of a mask having different interval sizes or different mask pattern densities, the light through the regions having small interval sizes is influenced by the transparent regions having large interval sizes, which may result in the deformation of the transferred pattern or a micro-loading effect.
To overcome the illustrated problems, in the prior art dummy patterns are disposed in the spaces between the transparent regions having different interval sizes, so that when the mask pattern is transferred to the photoresist layer, the pattern deviation caused by the OPE may occur in the dummy patterns only, and the integrity of the predetermined transferred pattern can be achieved. In another solution, scattering bars are disposed in the spaces between the transparent regions having different interval sizes, and the deposition of scattering bars prevents the transferred pattern corresponding to the transparent regions having small interval size from being affected by the OPE effect. Nevertheless, the dummy patterns will be transferred to photoresist layer, and the feasible utilization rate of the semiconductor wafer may decrease. Moreover, the deposition of scattering bars reduces the feasible utilization rate of the mask and increases the layout budget of mask. Consequently, when the mask includes the transparent regions having different interval sizes, a way to obtain the complete transferred patterns corresponding to the transparent regions having small interval size is still an important issue in the field.